In power audio amplifiers (provided in either a monolithic or discrete form), the presence of devices for protection against abnormal or dangerous operating conditions for the amplifier, such as, for example, excessive output current or temperature, is important.
An existing method of providing such protection includes automatically turning off circuit portions subject to damage as soon as the abnormal condition is detected. It is therefore necessary to equip the amplifier with a circuit that enables it to operate again when the dangerous condition ceases or is removed. Such an enable circuit removes the need for the direct intervention by the user to turn off and restart the amplifier.
It is becoming ever more common to also equip audio amplifiers with protection that acts during the start-up phase. Indeed, this phase is potentially critical for the amplifier because the partial polarization of the final stages, typical of the start-up transition, can make ineffective the standard protection, but nevertheless be dangerous for the amplifier.
This additional protection device checks the presence of any critical conditions before the final stages begin to be polarized in order to timely stop the start-up sequence if necessary.
FIG. 1 shows a block diagram of an existing protection circuit that meets the requirements set forth above and is commonly used to protect the final amplifier outputs stage of audio power amplifiers.
Through a type OR logic gate circuit F, circuit switching means G for turn-off of the stage is controlled by three distinct lines.
If by trigger circuit means designed to detect abnormal operating conditions, there is detected, for example during normal operation, an overcurrent in the output elements of the amplifier stage, there is generated a so-called trigger pulse that controls, directly through the logic gate F, the circuit means G for turn-off of the stage.
The same pulse is coupled to the input terminal S of a bistable, or flip-flop, circuit L.
The output Q of the flip-flop L also enables a sensing circuit B designed to detect the persistence of abnormal operating conditions to control or instruct through the logic gate F the circuit switching means G to keep the stage turned off.
The input terminal R, through which the flip-flop L is returned to the initial stable state, which is that of normal operating conditions, is connected to the output terminal of a logic gate circuit D of the AND type. Through this AND logic gate D, the flip-flop L can receive either a pulse which is the inverse of the trigger pulse or a signal generated by the sensing circuit B in phase opposition to that with which the sensing circuit B controls the circuit switching means G. Through the AND logic gate D and the flip-flop L, the sensing circuit B is disabled for keeping the stage turned off when persistence of the abnormal operating conditions ceases.
All the signals used to activate or deactivate the circuit elements just indicated are determined on the basis of the characteristics of the integrated circuit in which the protection device is included and the supply levels provided for the final stage.
Again through the type OR logic gate F, turn-off of the stage can be determined also by a second sensing circuit, indicated by E, which is enabled to detect the persistence of abnormal conditions only in the amplifier turn-on phase.
Suppose that the initial turn-on phase, which due to the presence of the active components cannot occur instantaneously, is determined by the application of a positive voltage step to the network RC of an interface of the so-called amplifier stand-by. The rising voltage across capacitor C1 is detected by a comparator A of the two-threshold, i.e., the so-called window type.
Comparator A enables the second sensing circuit E only when the voltage across C1 is between a lower voltage threshold and an upper voltage threshold.
The upper voltage threshold of this voltage window is chosen at a voltage level lower than that at which the amplifier is turned on, to permit the sensing circuit E to complete the checks before the amplifier is capable of operating.
In other existing amplifiers, the various circuit blocks of FIG. 1 are implemented in different manners that are known in the art.
When the amplifier has several output channels, there must be a pair of sensing circuits provided for each channel: one, B, for protection during normal operation, and one, E, for protection during turn-on.